The measurement of blood pressure is a well accepted procedure in many medical environments. These measurements typically comprise three parameters. The two most commonly used parameters are those referred to as systolic and diastolic pressure which are the maximum and minimum pressures, respectively, of the waveform. Mean arterial pressure is also used occasionally and provides an average reading of the pressure. In some situations, the continuous blood pressure waveform is also utilized. These measurements are often used for diagnosis of hypertension as well as general monitoring of patient status.
Apparatus for making these measurements fall into two basic categories, invasive and non-invasive devices. The most direct measurements are made by inserting an arterial catheter into the patient's artery. This method provides a very accurate measurement of blood pressure parameters and the blood pressure waveform at the site where the catheter is located. The pressure is measured in one of two ways. The original and most common method involves a thin cannula which provides a direct connection between the artery and a pressure measurement system. This method is attractive as it allows sampling of blood during operations and other procedures.
Another method utilizes a pressure transducer which is attached directly to the tip of the catheter with the signal transmitted electrically to the monitoring equipment. Despite the desirability of a direct blood pressure reading, this approach has a number of difficulties such as risk of infection, occlusion of the artery causing erroneous signals and serious artery damage, and risk of disconnection resulting in blood loss. As a result of these difficulties, arterial catheters are only used when absolutely necessary, for example, during major operating procedures and for some intensive care unit (ICU) patients where a continuous blood pressure signal for monitoring patient status is needed. Therefore, substantial opportunity exists for a non-invasive sensor that can provide a continuous blood pressure waveform.
Indirect blood pressure measurements are made with non-invasive sensors that may provide intermittent or continuous blood pressure measurements. Manual measurements are typically made through auscultation using a sphygmomanometer. This involves placing an occluding cuff around the patient's arm. The cuff is manually inflated using a separate bulb to a pressure which is sufficient to stop blood flow in the brachial artery. The operator uses a stethoscope to listen to the blood flow on the distal side of the cuff. The pressure in the cuff is slowly released. When the pressure in the cuff is equal to the systolic or maximum pressure, some blood will pass through the occluded artery. As this blood is forced through under pressure, squirting sounds known as Korotkoff sounds are produced which are heard through the stethoscope. By knowing the pressure in the cuff when the Korotkoff sounds are first heard, the systolic pressure can be derived. The pressure in the cuff is further reduced until the Korotkoff sounds disappear. This pressure is equivalent to the diastolic pressure since the artery is no longer being occluded.
While this method provides reasonably accurate systolic and diastolic measurements when used by an experienced operator, it is complicated to use. It also only provides intermittent readings of blood pressure and takes a lot of time to perform.
Numerous technologies exist to automate this measurement, the most popular of which are the automated auscultation and the oscillometric techniques. Automated auscultation is an automated version of the manual technique previously described. A microphone is placed underneath the cuff and an instrument senses the Korotoff sounds while inflating and deflating the cuff.
Oscillometry also uses an inflated occluding cuff but not the Korotkoff sounds. Instead, pressure sensors monitor the pressure inside the occluding cuff. When the pressure in the cuff is in between the systolic and diastolic arterial pressure, it partially occludes the blood flow and causes pressure changes in the artery to be transmitted into the cuff. Therefore, the onset and cessation of these pressure oscillations can be used to detect systolic and diastolic pressures respectively.
These techniques only provide intermittent readings of blood pressure parameters but take less time to conduct than the manual method using the sphygmomanometer. The accuracy of these products has also been questioned, particularly when the subject is moving. Motion drastically affects pressure in the cuff and, as such, affects the accuracy of the system. The accuracy of these methods has also been questioned when compared against direct measurements and manual auscultatory readings.
Several methods exist for the determination of continuous non-invasive blood pressure monitoring. However, none of these has been widely accepted. Problems include susceptibility to motion, questionable accuracy, and poor sensor locations.
Two recent methods include the finapres technology and arterial tonometry. The finapres method uses an active cuff attached to the patient's finger. The pressure in the cuff is adjusted by a servo mechanism so that it is equal to the arterial pressure. This can be achieved by keeping the volume of the artery fixed though a light source and a sensor transilluminating the finger. By maintaining this cuff pressure equal to the arterial pressure, a pressure transducer in the cuff provides a reading of the arterial pressure. While quite sophisticated, the accuracy of this method is susceptible to motion in the cuff which is a problem due to the fact that the finger is easily moved. Measurements made on the finger are also subject to substantial changes depending on the position of the finger relative to the heart and also to changes in the blood flow during changes in physiologic state.
Arterial tonometry places a piezoelectric film in close contact with the arteries in the wrist in order to monitor deflections in these arteries as a result of beat-to-beat changes in blood pressure. A piezoelectric film generates a voltage as a result of mechanical changes. By placing such a film in close contact with the arteries in the wrist, an electrical signal is generated that is representative of the arterial blood pressure waveform. This waveform is calibrated against an absolute measure of blood pressure to obtain systolic and diastolic readings from each cycle of the waveform. This method is also subject to inaccuracies due to motion and position of the arm, as well as questionable accuracy when compared against more accurate methods.
Many other technologies have been employed in an attempt to derive a continuous non-invasive blood pressure waveform. These include brachial artery stress-strain which monitors continuous pressure oscillations transmitted from the artery to a partially inflated cuff. Bioimpedance has also been used to monitor changes in the body's impedance as blood pressure changes. Finally, pulse time transit delay has been used to determine the difference in time of arrival of pressure waves at different sites in the body, for example, at the two ear lobes.
In a search for literature related to the subject matter of the present invention, the following documents were found, some of which disclose some of the aforementioned prior art methods. U.S. Pat. No. : 5,261,412 to Butterfield; U.S. Pat. No. 5,237,997 to Gruefbel; U.S. Pat. No. 5,163,438 to Gordon; U.S. Pat. No. 5,158,091 to Butterfield; U.S. Pat. No. 5,152,296 to Simons; U.S. Pat. No. 5,025,792 to Hon; U.S. Pat. No. 4,960,128 to Gordon; U.S. Pat. No. 4,846,189 to Sun; U.S. Pat. No. 4,807,638 to Sramek; U.S. Pat. No. 4,718,426 to Russell; U.S. Pat. No. 4,718,428 to Russell; U.S. Pat. No. 4,669,485 to Russell; U.S. Pat. No. 4,677,984 to Sramek; U.S. Pat. No. 4,425,920 to Bourland and U.S. Pat. No. 4,406,289 to Wesseling.